BLIGHT PREVENTION
By Eric Sideman, Ph.D.
As everyone knows, last summer was wet and late blight was widespread on farms and in gardens. Some of you may be tired of hearing about it, but whether the crisis repeats in 2010 depends greatly on the weather and on what gardeners and farmers do to prevent the disease.
I have repeatedly stressed that understanding the biology of the organism that causes the disease is the first step in avoiding it. Late blight is caused by a fungus with great similarity to some algae, which helps explain why it needs lots of moisture to do well. The fungus is called Phytophthora infestans, which may sound familiar to those who remember history, because this organism was responsible for widespread potato problems in Ireland in the mid-1800s. Late blight occurs regularly in Maine, but usually it is spotty and not severe – and usually it does not affect tomatoes. Let’s look at what differed in 2009, and what can we do in 2010.
LIFE CYCLE
In the typical life history of late blight (Figure 1, below), the pathogen persists from year to year only in potato tubers and usually spreads from sprouting tubers in cull piles; from volunteer potato sprouts from culls left in the garden or field while harvesting; and, less commonly, from planting diseased seed. The diagram shows that when diseased potato pieces sprout in the spring, the young plant carries the disease, which eventually releases sporangia (spore-containing structures). These sporangia land on new potato or tomato leaves, germinate and infect the new plant, and then this diseased plant produces more sporangia that infect more plants.
The sporangia can blow in the wind for 50 miles. If conditions are damp and cool, late blight can spread fast and far.
Usually conditions are not right. The spores are actually quite wimpy and die within hours in sunlight or dry conditions. We had very little sunlight and dry conditions the first half of last summer, so late blight spread like wildfire.
[Note that two mating types, A1 and A2, are required for sexual reproduction of the late blight fungus. So far in Maine and in most of the United States, only one mating type occurs at a time in any location. For more than a decade, only mating type A2 has been found in Maine. Both occur together in Mexico. Let’s hope both never occur together in Maine. When both do occur together, sexual reproduction takes place, producing a different kind of spore (an oospore) that is very hardy and resistant to drying, cold, heat, etc.]
CONTROLS
The late blight fungus is an obligate parasite, i.e., it survives only on living tissue. In the Northeast, very little plant tissue survives the winter. Except in greenhouses, only potato tubers live through winter. Since the spores are wimpy, growers need only pay attention to managing potato tubers. If all growers adhere to the following steps, we should avoid the strain of late blight that invaded us from the south last year.
1. Do not keep cull piles of potatoes.
2. Do not save questionable potato seed.
3. Do not compost diseased tubers, because parts of the compost pile may not be hot enough to kill the tubers but may be warm enough so that the tubers don’t freeze in the winter, providing the pathogen with living tissue for overwintering.
4. Buy seed from a good source.
5. In the spring, scout, pull and destroy all volunteer potatoes.
People often ask me if they should clean tomato cages and whether tomato seed carries late blight. I recommend cleaning cages, stakes, etc., but not for late blight. Late blight will not survive there because all the plant tissue on the cages is dead; but other common tomato diseases do survive, so yes, clean the cages.
Here’s the good news: Late blight is not carried on tomato seed.
Eric Sideman is the Maine Organic Farmers and Gardeners Association (MOFGA's) organic crops specialist. You can address your questions to him at 207-568-4142 or esideman@mofga.org
